Heat transfer surface having a high boiling heat transfer coefficient

ABSTRACT

A heat transfer surface comprising a heat conductive base member and a plurality of spaced apart fins extending therefrom, each of said fins being bent over towards the next adjacent fin so as to form therebetween elongated, re-entrant cavities. The gap between the tip of one of said fins and the next adjacent fin is sufficiently small to promote and sustain nucleate boiling of a fluid in contact with said fins and said base member.

United States Patent Webb [ 51 Oct. 10,1972

[54] HEAT TRANSFER SURFACE HAVING A HIGH BOILING HEAT TRANSFER COEFFICIENT [72] Inventor: Ralph L. Webb, La Crosse, Wis.

[73] Assignee: The Trane Company, La Crosse,

Wis.

[22] Filed: May 18, 1970 [21] Appl. No.: 37,476

Related US. Application Data [63] Continuation of Ser. No. 657,155, July 31,

1967, abandoned.

[52] US. Cl. ..l65/l33, 62/527, 165/184, 165/1, 29/1573 [51] Int. Cl. ..F28f 13/00 [58] Field of Search ..165/184, 133,185,18l,1; 62/515, 527

[56] References Cited UNITED STATES PATENTS 3,454,081 7/1969 Kunetal. .165/18 5X 3,457,990 7/ 1967 Theophilos et al. ...165/ 185 X 3,299,949 1/1967 Beurtheret 1 65/1 85 3,454,081 7/1969 Kun et a1. 1 65/105 3,326,283 6/1967 Ware ..165/185 X FOREIGN PATENTS OR APPLICATIONS 1,226,420 2/1960 France 165/1 85 Primary Examiner--Albert W. Davis, Jr. Attorney-Arthur 0. Andersen and Carl M. Lewis 7] ABSTRACT A heat transfer surface comprising a heat conductive base member and a plurality of spaced apart fins extending therefrom, each of said fins being bent over towards the next adjacent fin so as to form therebetween elongated, re-entrant cavities. The gap between the tip of one of said fins and the next adjacent fin is sufficiently small to promote and sustain nucleate boiling of a fluid in contact with said fins and said base member.

15 Claims, 6 Drawing Figures P'A'T ENTEDHBI 1 Ian 3.696.861

SHEET 1 UF 2 INVENTOR. RALPH L. WEBB BYM QW ATTORNEY HEAT TRANSFER SURFACE HAVING A HIGH BOILING HEAT TRANSFER COEFFICIENT CROSS REFERENCE TO RELATED APPLICATION BACKGROUND OF THE INVENTION One of the highly effective ways of transferring heat I from a heated wall to a fluid in contact therewith is through the mechanism of nucleate boiling. According to the most commonly accepted theory of nucleate boiling, irregularities or cavities in the heat transfer surface known as nucleation sites trap minute amounts of vapor whichform the nucleus of a bubble. The bubbles grow and detach from the surface as the liquid on the surface is heated above its saturation temperature. In-

cipient boiling or initial significant bubble formation requires that the nucleation sites be covered by a certain thickness of superheated liquid. As the bubbles rise in continuous columns from nucleation sites, they interrupt the boundary layer of superheated liquid and carry superheated liquid away from the hot wall surface. It is believed that the greater part of vapor formation in nucleate boiling occurs as superheated liquid evaporates into the liquid-vapor interface of rising bubbles. Moreover, the agitation of the liquid by rapid bubble departures increases the rate of heat transfer to the liquid by forced convection.

These advantageous heat transfer effects associated with the bubble columns indicate that surface heat transfer should be high in the locality of the bubble emission sites. It may then be assumed, as a natural corollary, that the heat transfer rate, and especially the boiling heat transfer rate, will increase in direct proportion to the number of active bubble column sites. My experimental results, as well as the work of others, have established that this is indeed the case. See for example, H. M. Kurihari and J. E. Myers, The Effects of Superheat and Surface Roughness on Boiling Coefficients, American Institute of Chemical Engineers Journal, Vol. 6, No. 1, pp. 8391 (1960). Thus, for a given temperature differential (AT) between the temperature of a hot wall (Tw) and the saturation temperature of a liquid (Ts) in contact therewith, the boiling heat transfer coefficient will vary with the area density of nucleation sites. It would therefore be highly desirable from a performance standpoint to treat or condition a heat transfer surface in such a way as to cause a greater density of bubble columns for a particular value of AT.

It is known that artificial nucleation sites can be created in a surface by proper surface conditioning techniques. For examples of such techniques see, J .A. Clark, Theory and Fundamental Research in Heat Transfer, p. 64, Pergamon Press (1963) and US. Pat. No. 3,301,314 to Gaertner. Surfaces which have been roughened so as to produce a large number of discrete pits, scratches or cavities of microscopic size which act as good vapor traps have been found to be good nucleate boiling surfaces. The problem in attempting to utilize the mechanism of nucleate boiling on a commercial basis lies in arriving at a particular surface geometry which has a large population density of nucleation sites (100 to 200 per square inch), and which can be consistently and economically reproduced in relatively large quantities.

. 2 BRIEF SUMMARY OF THE INVENTION In the course of working on the aforesaid problem of finding a commercially useful nucleate boiling surface, 1 have developed a heat exchange structure which has a high boiling heat transfer coefficient and which is quite simple and inexpensive to produce. In contrast with prior art boiling surfaces having a number of discrete nucleation sites as disclosed in 1.1.8. Pat. No. 3,301,314, my improved surface is comprised of plurality of spaced apart, elongated grooves shaped to act as good vapor traps. The grooves are formed by providing a plurality of fins on a base member and bending the fins over on each other. The fins are bent in such a manner and to such extent that the tip of each fin is spaced from the next adjoining fin by a gap which is narrower than the space between the bases of said fins. The aforesaid gap is controlled so as to be small enough to initiate nucleate boiling, and is preferably between 0.0015 and 0.0035 inches wide where the boiling fluid is Refrigerant 11. By bending each fin over so that it is curved from its base to its tip, with its tip overlapping the base of the next adjacent fin, an elongated, smoothly contoured, re-entrant shaped groove is provided between adjacent fins. These grooves serve as excellent vapor traps and have demonstrated an exceptional propensity for generating bubble columns over their entire length.

In the preferred form of my invention, the aforesaid elongated, nucleation promoting grooves are produced by bending the fins over on a finned tube of the well known type commonly employed in shell-and-tube heat exchangers.

These and other features of my improved boiling surface are described in detail below with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front elevation view of a finned tube showing a number of the fins shaped to provide the nucleate boiling surface of my invention.

FIG. 2 is a vertical section view taken along line 2-2 of FIG. 1.

FIG. 3 is a vertical section view on an enlarged scale taken along line 3-3 of FIG. 2.

F IG. 4 is a diagrammatic view of a refrigeration system including an evaporator in which my improved nucleate boiling surface could be employed.

FIG. 5 is a graph showing the improved heat transfer performance obtained with a surface conditioned in accordance with this invention.

FIG. 6 is a schematic illustration of a machining arrangement for forming the heat transfer surface geometry of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates the manner in which my nucleate boiling groove concept can be applied to a finned tube. A plurality of spaced apart fins 2 extend from the base member or tube 1, and may be connected in a continuous helical pattern as in the configuration shown. Fins 2 could be made from a separate material and attached to the outer surface of tube 1 or they could be machined from tube 1 so as to be integral therewith. The latter arrangement has been shown for illustrative purposes. The substantially vertically extending fins 2 on the left side of tube 1 represent a conventional finned tube surface prior to forming according to my invention.

I have found that by bending fins 2 over on eachother so that they assume the shape shown on the right side of tube 1 inFIG. 1, and inFIGS. 2 and 3, the finned tube surface will exhibit very high heat transfer coefficients in nucleate boiling. There are several significant features of the resulting surface geometry which I believe are largely responsible for this improved boiling performance. First, tip portions 4 of each fin 2 are bent over so far that their outer edges 6 are separate from the next adjoining fin by a narrow gap a which is of nucleate boiling size range. The necessary size range for gap a will vary with the particular fluid boiling on the outer surface of finned tube 1.

My experimental pool boiling results with Refrigerant ll (Trichloromonofluoromethane) at a saturated boiling temperature of from 40 to 70 F. have demonstrated that the boiling heat flux drops off rapidly when gap a is outside of the range of 0.001 to 0.005 inches and my preferred range for gap a is 0.0015 to 0.0035 inches.

The saturated nucleate boiling obtained from fins formed as shown in the right portion of FIG. 1 and having a gap in the range of 0.001 0.005 will provide a substantially higher heat flux at a given AT than would be provided by fins formed as shown in the left portion of FIG. 1.

This substantially increased performance has been proved for a substantial number of refrigerants including R-l 1, R-l2, R-22, R-113, R-ll4 and R-502, i.e., trichlorofluorornethane, dichlorodifluoromethane, monoch-lorodifluoromethane, trichlorotrifluoroethane, dichlorotetrafluoroethane, and a 48.8/51.2 weight percent azeotropic mixture of monochlorodifiuoromethane and monochloropentafluoroethane, respectively.

A second requisite characteristic of the final fin geometry is that the distance of gap a between the tip edge 6 of one fin andthe next adjacent fin be less than the distance between base portions 8 of adjacent fins 2. This insures that a re-entrant shaped cavity or groove 10 having a restricted opening defined by gap a is formed between adjacent fins 2. Additionally, fins'2 are bent from their bases 8 so that bases 8 intersect tube 1 at an angle to the vertical. This permits tip portions 4 to be bent over sufficiently far that their outer edges 6 overlap the base 8 of the next adjacent fin by a distance b shown in FIG. 3. I prefer that the dimension b shall be in the range of one-half to one and one-half times the thickness of the fin. These latter two geometrical features, coupled with the fact that fins 2 are curved over their entire height, provide a smoothly contoured, re-entrant groove 10 which gradually narrows towards outlet a, as shown in-cross-section in FIG. 3. Grooves 10 are thus of such a shape as to present a smooth, direct, unimpeded path for the growth of bubbles within said groove and for the discharge of bubbles out of gap a.

When the basic surface to be modified in accordance with my invention is a tube, fins 2 may be'rollformed from the tube outer surface in a continuous helix in a manner well known in the art. Fins 2 may then be bent over to provide the above described elongated re-entrant grooves 10 in a simple operation as illustrated in FIG. 6. I have accomplished this by rotating a finned tube 1 in a chuck and feeding a bending or rolling tool 14 parallel to the tube axis by means of a lead screw. The folling or bending tool must have an angled tip 16 capable of producing the desired degree of bending of the fins. The direction of rotation of tube land the direction of movement of tool 14 are indicated by arrows in FIG. 6.

' Finned surfaces having differing numbers of fins per inch and various fin thicknesses and heights have been processed to. provide re-entrant grooves of the aforesaid geometry and have demonstrated significantly improved performance in nucleate boiling. For example, substantially the same results were achieved with a finned surface having 33 fins per inch, with fins 0.030 inches high and 0.010 inches thick as with a finned surface having twice as many fins per inch with fins one-half as high and thick. The size of gap a between bent over fins and the re-entrant shape of elongated grooves 10, whichare believed to be the most critical factors, were held the same in both cases.

The performance curves of FIG. 5 illustrate the improved heat transfer achieved with my novel surface. These results were obtained in a pool boiling environment using Refrigerant 11 at a saturated pool boiling temperature of F. Curve I shows the boiling performance of an ordinary finned surface as illustrated on the left side of the tube of FIG. 1. Curve II shows the boiling performance achieved with fins 2 bent over to form re-entrant grooves 10 as illustrated on the right side of the tube in FIG. 1 and in FIGS. 2 and 3. The dramatic improvement in boiling performance obtained with my bent fin arrangement is readily apparent. For example, with a AT of 4, the total boiling heat flux in Btu per hour per square foot of base surface for my improved boiling surface was 1 1,000 compared to a value of only 1,750 for the ordinary finned surface. These results are especially significant when it is borne in mind that the only physical difference in the two surfaces is that the fins of the curve II surface were rolled over on each other to form grooves 10 of the shape described above. The test data for the curves of FIG. 5 was obtained with a copper surface. Equally significant results were expected with other metals, although performance may varyslightly with different fluid-surface combinations.

For applications such as the cooling of electronic apparatus and nuclear reactors where heat is dissipated directly from a heat source through a wall to a boiling fluid such as water, the utilization of my bent fin surface could provide a very great increase in heat flux on I An example of such an application would be a refrigerant evaporator of the shell-and-tube type. A fluid to be cooled, such as water, is normally directed through the inside of tubes which transfer heat from the water to refrigerant evaporating on the outside of the tubes. Since the overall heat transfer coefficient is then comprised of the combination of the coefficients for the water and refrigerant sides of the tubes, and only the refrigerant side coefficient is improved by my boiling surface, only a limited increase in total heat flux can be realized.

FIG. 4 illustrates diagrammatically a standard compression refrigeration system with a shell-and-tube evaporator 20 in which my bent fin tube surface could be used. Evaporator 20 is connected in a refrigeration circuit including compressor 22, condenser '24, and flow regulating valve 26. Either a reciprocating or centrifugal type of compressor could be employed, centrifugal compressor 22 having been shown for illustrative purposes. Evaporator 20 is comprised of a shell 21, headers 23 and 25, and closely spaced tubes 30 for conducting fluid to be cooled from inlet header 23 to outlet header 25. Water or other fluid to be cooled flows from inlet 28 through tubing 30 and is discharged through outlet 32. Refrigerant liquid from condenser 24 is expanded into shell 21 as it flows from control valve 26. The refrigerant which enters evaporator 20 is a mixture of liquid and vapor. The liquid is evaporated as the refrigerant flows through shell 21 in contact with the outside of tubing 30. Heat transfer to the refrigerant thus takes place by the combined modes of forced convection and nucleate boiling, thus making it more difficult to predict the total increase in heat flux to be realized by improving the nucleate boiling performance of finned tubing 30. My experimental results have demonstrated that a significant increase in total heat flux is achieved by utilizing my bent fin surface unser such conditions. The net increase in heat flux closely approximates the direct addition of the pool boiling and forced convection heat fluxes for a particular surface.

The precise mechanism which operates to improve the nucleate boiling performance of my bent fin surface is difficult to define with certainty. Two possible theories present themselves. First, the high boiling performance may be due at least in part to modifications of the hydrodynamic conditions in the vicinity of preexisting nucleation sites in the form of pits and scratches on tube 1 and the walls of fins 2. Nucleate boiling theory assumes that when a bubble departs from a surface, liquid surrounding the nucleation site rushes in to fill the void left by the departed bubble. The site will not again activate until this liquid is warmed up to the necessary superheat level. With ordinary finned tubes, or with prior art boiling surfaces having a plurality of surface pits, cold, saturated liquid from above the fins or base surface rushes directly onto the fully exposed nucleation sites. A relatively large heat flux is then required to bring this cold liquid to the incipient boiling point. My bent fin surface, by contrast, causes superheated liquid to flow onto the active sites on the top surface 11 of tube 1 and on the side walls of fins 2. This is due to the large hydraulic resistance imposed by narrow fin gap a which tends to restrict the flow of cold liquid from above fins 2 down onto tube 1. Cold liquid from above fins 2 seeps into groove 10 along the length of gap a between rising bubble columns, and then flows along groove 10 toward nucleation sites. As it flows along groove 10, the liquid will be heated by rolled over fins 2 so that it is brought to a superheated condition by the time it reaches the active sites. Thus, only a small amount of heat need be added at each nucleation site to further raise the temperature of the liquid to the level required for nucleation. By channeling and directing superheated liquid rather than cold, saturated liquid onto sites from which a bubble has departed the rolled over fin arrangement appreciably decreases the heat flux required to form and release another bubble. The nucleate boiling rate is thus increased.

Although the aforesaid action of the grooves 10 in improving the boiling performance of small imperfections on the tube and fins is no doubt helpful, this mechanism is not believed to be the dominant factor providing the overall improvement in heat transfer. The second and more significant boiling mechanism believed to be present derives from the formation and growth of bubbles directly under bent over fin tips 4 along the length of elongated grooves or cavities 10. The re-entrant shape of grooves 10 makes them good vapor traps. Observation of the boiling action indicates that golbules of vapor are trapped at spaced intervals along grooves 10. The curved shape of fins 2 and the large degree to which they are bent over from their bases provides a gradually increasing volume within grooves 10, thereby permitting the liquid-vapor interface 12 (FIG. 3) of trapped bubbles or vapor globules to develop a relatively large radius of curvature. This is important because it is generally recognized that the larger the radius of curvature of a bubble, the lower the liquid superheat required to make it grow. Bubble growth may thus take place within grooves 10 with very small superheat values. Bubble growth and departure is accelerated by the presence of a relatively large mass of superheated liquid which is retained under fins 2 along the bottom of grooves 10 and rapidly evaporates into liquid-vapor interface 12. The superheated liquid trapped within grooves 10 is sheltered from the cold, saturated liquid above fins 2 by narrow gaps a. This is a distinct advantage over prior art boiling surfaces where bubbles grow upwardly from discrete surface cavities into a very thin layer of superheated liquid which is susceptible to mixing with cold liquid and which is periodically destroyed as bubbles depart. As evaporation into a trapped bubble proceeds, the bubble grows downwardly and longitudinally within a groove 10 until the buoyancy forces overcome the surface tension along gap a. The bubble then breaks loose from groove 10 and escapes outwardly through gap a. It is to be noted here that the size of gap a is critical, because if it is too small excessive superheat would be required for the existence of a bubble within the nar row groove. On the other hand, if gap a is too large, grooves 10 will not act as good vapor traps.

It appears that a substantial mass of vapor remains within elongated cavities 10 after a bubble is emitted. Immediately after bubble departure, liquid from above the rolled over fins 2 rushes in to fill the void left by the departed bubble, and due to its inertia, the partially penetrates into grooves 10 through gaps a. This violent liquid motion causes the trapped vapor residue to move along grooves 10 at a high velocity. As this liquid-vapor interface moves longitudinally within a groove 10, superheated liquid retained within the groove evaporates into it until additional bubble growth causes the detachment of another bubble. In

this way a new bubble column is started at a point along a groove 10 spaced from theinitial bubble departure point. Ultimately a series of closely spaced bubble columns is generated along the whole length of each groove 10. Adjacent bubble columns showed a predominant characteristic of having the same bubble departure frequency but different phases. This tends to confirm the existence of an oscillating liquid-vapor interface moving back and forth between adjacent bubble sites with bubble growth and departure in phase with the oscillations. Thevapor moving aside from one site within a groove 10 as a bubble departs therefrom appears to trigger the growth and departure of another bubble at an adjacent site. Thus, in addition to retaining superheated-liquid which promotes rapid bubble growth, grooves 10 serve the very useful purpose of placing adjacent bubble column sites in direct fluid communication.

My bent fin surface achievesthe desirable goal of substantially improving the nucleate boiling performance of the widely utilized finned tube. This has been accomplished ina very simple way requiring only the rolling over of the fins in the manner described above. Although I have shown and described the rolled over fin configuration as applied to a tubular base membenit could readily be formed on a flat plate as well. Whether the base member is a tube or a plate, there are obviously a great variety of heat transfer ap plications on which the unique boiling surface geometry of this invention could be applied.

I anticipate that those skilled in the art may think of variations and modifications of my re-entrant shaped groove arrangement which will be within the spirit and scope of my invention as defined by the following claims.-

lclaim:

l. A heat exchange comprising: a heat conductive base member for transferring heat from a heat source on one side thereof to a boiling fluid on the other side thereof; a plurality of spaced apart fins having substantially smooth and uninterrupted side surfaces extending from said other side of said base member, each of said fins having a base portion joined to said base member and a tip portion, said tip portions being bent over toward the next adjacent one of said fins to form a a continuous gap having a width of from 0.001 to 0.005 inches, the gaps between said tip portions and said next adjacent one of said fins being less than the spaces between said base portions, whereby a continuous reentrant shaped cevity sufficient to promote nucleate boiling of a given liquid is formed between adjacent ones of said fins; said boiling fluid selected from the group of refrigerant fluids consisting of trichloromonofluoromethane, monochlorodifluoromethane, dichlorodifluoromethane, trichlorotrifluoroethane, dichlorotetrafluoroethane and a 48.8/5l.2 weight percent azeotropic mixture of monochlorodifluoromethane and monochloropentafluoroethane, respectively.

2. A heat exchanger as defined in claim 1 wherein:

said bent fins are of curvilinear cross-section over substantially their entire height.

3. A heat exchanger as defined in claim 1 wherein:

said tip portions overlap said base portion of saidnext adjacent one of said finssby a distance of from one-half to one and one-half times the thickness of one of said fins.

4. A heat exchanger as defined in claim 1 wherein:

said boiling fluid comprises trichloromonofluoromethane and said gaps between said tip portions and said next adjacent one of said fins are within a range of from 0.0015 to 0.0035 inches.

5. A heat exchanger as defined in claim 1 wherein:

said fins are formed integrally with said base member.

6. A heat exchanger as defined in claim 1 wherein:

said tip portions overlap said base portion of said next adjacent fins.

7 In a refrigeration system comprising a compressor, condensor, pressure reducing means and an evaporator of the shell-and-tube type interconnected in refrigerant flow relationship, an improved heat transfer surface for said evaporator comprising: a plurality of tubular members through which a relatively warm fluid to be cooled passes; a plurality of spaced apart fins having substantially-smooth and uninterrupted side'surfaces extending from each of said tubular members, the outside of said tubular members and said fins being in contact with refrigerant fluid flowing through said evaporator; and each of said fins having a base portion joined to one. of said tubular members and a tip portion, each of said tip portions being bent over toward .the next adjacent one of said fins to overlap said base portion of said next adjacent one of said fins and being separated therefrom by a gap sufficiently narrow to promote and sustain nucleate boiling of said refrigerant fluid; said boiling fluid selected from the group consisting of trichloromonofluoromethane, monochlorodifuloromethane, dichlorotetrafluoroethane and a 48.8/51.2 weight percent azeotropic mixture of monochlorodifuloromethane and monochloropentafluoroethane, respectively; said gaps between said tip portions and said next adjacent one of said fins being in the range of from 0.001 to 0.005 inches.

8. Apparatus as defined in claim 7 wherein: each of said fins is curved over its entire height towards said next adjacent one of said fins. 9. Apparatus as defined in claim 7 wherein: said refrigerant fluid comprises trichloromonofluoromethane and said gap is between 0.0015 and 0.0035 inches wide. 10. A heat exchanger comprising a plurality of tubes for conducting a relatively warm fluid to be cooled by transferring heat to a boiling fluid surrounding said tubes, helical heat transfer fins on said tubes and ex-v tending outwardly from said tube, said helical heat transfer fins having base portions integral with the outer surface of said tube and having substantially smooth and uninterrupted side surfaces and tip portions, said tip portions being bent over toward the next adjacent one of said heat transfer fins, the spaces between said heat transfer fins at their outer ends being less than the spaces between said. heat transfer fins at their bases whereby continuous re-entrant shaped cavities are provided to enhance boiling.

11. A heat exchanger as defined in claim 10 wherein the spaces between the fins are of substantially uniform cross-section along substantially the length of the fins.

12. A heat exchanger comprising a plurality of tubes for conducting a relatively warm fluid to be cooled by transferring heat to a boiling fluid surrounding said tubes, helical heat transfer fins on the outer surface of said tubes, said helical fins having base portions integral with the outer surface of said tubes, said fins having substantially smooth and uninterrupted side surfaces and extending outwardly from their base portions to distal portions, the distal portions being curved toward the next adjacent fins, the spaces between the distal portions and the next adjacent fins being less than the spaces between the base portions of the fins whereby substantially continuous re-entrant shaped cavities are provided to enhance boiling.

13. A heat exchanger as defined in claim 12 wherein the spaces between the helical heat transfer fins at their outer ends are substantially uniform in width along substantially the length of the fin.

14. A heat exchanger as defined in claim 12 wherein said helical heat transfer fins are curved in cross-section over substantially their entire height.

15. A heat exchanger comprising a tube for conducting a relatively warm fluid to be cooled by transferring heat to a boiling fluid surrounding said tube, helical heat transfer fins at the outer surface of and substantially co-axially disposed with respect to said tube, said helical fins having base portions integral with the outer surface of said tube, said fins having substantially smooth and uninterrupted side surfaces and extending outwardly from their base portions to distal portions, the distal portions being curved toward one end of the tube and terminating in closely spaced relation with the next adjacent fin to define substantially uniform helical gaps between the distal portions and the next adjacent fins which are substantially less in width than the spaces between the base portions of the fins whereby substantially continuous helically shaped re-entrant cavities are provided to enhance boiling. 

1. A heat exchange comprising: a heat conductive base member for transferring heat from a heat source on one side thereof to a boiling fluid on the other side thereof; a plurality of spaced apart fins having substantially smooth and uninterrupted side surfaces extending From said other side of said base member, each of said fins having a base portion joined to said base member and a tip portion, said tip portions being bent over toward the next adjacent one of said fins to form a a continuous gap having a width of from 0.001 to 0.005 inches, the gaps between said tip portions and said next adjacent one of said fins being less than the spaces between said base portions, whereby a continuous reentrant shaped cevity sufficient to promote nucleate boiling of a given liquid is formed between adjacent ones of said fins; said boiling fluid selected from the group of refrigerant fluids consisting of trichloromonofluoromethane, monochlorodifluoromethane, dichlorodifluoromethane, trichlorotrifluoroethane, dichlorotetrafluoroethane and a 48.8/51.2 weight percent azeotropic mixture of monochlorodifluoromethane and monochloropentafluoroethane, respectively.
 2. A heat exchanger as defined in claim 1 wherein: said bent fins are of curvilinear cross-section over substantially their entire height.
 3. A heat exchanger as defined in claim 1 wherein: said tip portions overlap said base portion of said next adjacent one of said fins by a distance of from one-half to one and one-half times the thickness of one of said fins.
 4. A heat exchanger as defined in claim 1 wherein: said boiling fluid comprises trichloromonofluoromethane and said gaps between said tip portions and said next adjacent one of said fins are within a range of from 0.0015 to 0.0035 inches.
 5. A heat exchanger as defined in claim 1 wherein: said fins are formed integrally with said base member.
 6. A heat exchanger as defined in claim 1 wherein: said tip portions overlap said base portion of said next adjacent fins. 7 In a refrigeration system comprising a compressor, condensor, pressure reducing means and an evaporator of the shell-and-tube type interconnected in refrigerant flow relationship, an improved heat transfer surface for said evaporator comprising: a plurality of tubular members through which a relatively warm fluid to be cooled passes; a plurality of spaced apart fins having substantially smooth and uninterrupted side surfaces extending from each of said tubular members, the outside of said tubular members and said fins being in contact with refrigerant fluid flowing through said evaporator; and each of said fins having a base portion joined to one of said tubular members and a tip portion, each of said tip portions being bent over toward the next adjacent one of said fins to overlap said base portion of said next adjacent one of said fins and being separated therefrom by a gap sufficiently narrow to promote and sustain nucleate boiling of said refrigerant fluid; said boiling fluid selected from the group consisting of trichloromonofluoromethane, monochlorodifuloromethane, dichlorotetrafluoroethane and a 48.8/51.2 weight percent azeotropic mixture of monochlorodifuloromethane and monochloropentafluoroethane, respectively; said gaps between said tip portions and said next adjacent one of said fins being in the range of from 0.001 to 0.005 inches.
 8. Apparatus as defined in claim 7 wherein: each of said fins is curved over its entire height towards said next adjacent one of said fins.
 9. Apparatus as defined in claim 7 wherein: said refrigerant fluid comprises trichloromonofluoromethane and said gap is between 0.0015 and 0.0035 inches wide.
 10. A heat exchanger comprising a plurality of tubes for conducting a relatively warm fluid to be cooled by transferring heat to a boiling fluid surrounding said tubes, helical heat transfer fins on said tubes and extending outwardly from said tube, said helical heat transfer fins having base portions integral with the outer surface of said tube and having substantially smooth and uninterrupted side surfaces and tip portions, said tip portions being bent over toward the next adjacent one of said heat transfer fins, the spAces between said heat transfer fins at their outer ends being less than the spaces between said heat transfer fins at their bases whereby continuous re-entrant shaped cavities are provided to enhance boiling.
 11. A heat exchanger as defined in claim 10 wherein the spaces between the fins are of substantially uniform cross-section along substantially the length of the fins.
 12. A heat exchanger comprising a plurality of tubes for conducting a relatively warm fluid to be cooled by transferring heat to a boiling fluid surrounding said tubes, helical heat transfer fins on the outer surface of said tubes, said helical fins having base portions integral with the outer surface of said tubes, said fins having substantially smooth and uninterrupted side surfaces and extending outwardly from their base portions to distal portions, the distal portions being curved toward the next adjacent fins, the spaces between the distal portions and the next adjacent fins being less than the spaces between the base portions of the fins whereby substantially continuous re-entrant shaped cavities are provided to enhance boiling.
 13. A heat exchanger as defined in claim 12 wherein the spaces between the helical heat transfer fins at their outer ends are substantially uniform in width along substantially the length of the fin.
 14. A heat exchanger as defined in claim 12 wherein said helical heat transfer fins are curved in cross-section over substantially their entire height.
 15. A heat exchanger comprising a tube for conducting a relatively warm fluid to be cooled by transferring heat to a boiling fluid surrounding said tube, helical heat transfer fins at the outer surface of and substantially co-axially disposed with respect to said tube, said helical fins having base portions integral with the outer surface of said tube, said fins having substantially smooth and uninterrupted side surfaces and extending outwardly from their base portions to distal portions, the distal portions being curved toward one end of the tube and terminating in closely spaced relation with the next adjacent fin to define substantially uniform helical gaps between the distal portions and the next adjacent fins which are substantially less in width than the spaces between the base portions of the fins whereby substantially continuous helically shaped re-entrant cavities are provided to enhance boiling. 